platform/x86: ideapad-laptop: always propagate error codes from device attributes' show() callback

Consumers can differentiate an error from a successful read much more
easily if the read() call fails with an appropriate errno instead of
returning a magic string like "-1". This introduces an ABI change, but
not many users are expected to be relying on the previous behavior,
and this change makes this module conforming to the standard behavior
that sysfs attribute show/store callbacks return an appropriate
errno in case of failure. Thus the ABI breakage is deemed justified.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Link: https://lore.kernel.org/r/20210203215403.290792-15-pobrn@protonmail.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>